Stabilized tuned radio frequency amplifier



Sept. 12, 1939.

B. SALZBERG 2,172,454

STABILIZED TUNED RADIO FREQUENCY AMPLIFIER Filed Jan. 7, 1937 PHASE 0; g 0F AMPLIFIER syn-r 5 r "EL FROM 0 4 i 4 I L0 IQ INVENTOR BERNARD SALZBERG F /KZW ATTORNEY Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE Bernard Salzberg,

East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 7, 1937, Serial No. 119,462

7 Claims.

My present invention relates to tuned high frequency amplifiers, and more particularly to a novel method of, and means for, stabilizing tuned radio frequency amplifiers whose input and output circuits are resonated to the same radio frequency.

One of the main objects of my present invention may be stated to reside in the provision of a radio frequency amplifier having input and output circuits resonated to a common radio frequa-ncy, and the constants of the amplifier tube being chosen in such a manner that there is produced a phase shift in the transconductance, or transfer admittance, of the tube to an extent sufficient to minimize radio frequency feedback through the input grid-output electrode capacitance whereby the stability of the amplifier stage is greatly improved.

Another important object of the invention may be stated to reside in the provision of a tuned radio frequency amplifier stage, capable of operating over a wide range of radio frequencies with substantial stability, the amplifier tube being so constructed that the transit angle between the screen grid thereof and the suppressor grid is sufficiently large to produce a substantial phase shift in the amplifier tube transconductance with the result that the radio frequency feedback through the grid to plate capacitance is substantially prevented. The expression transit angle is defined as 21r times the ratio of the time of flight of an electron between any two electrodes of a tube (say from the screen grid to the suppressor grid) to the period of the alternating current being amplified; this definition holds regardless of what electrode current is referred to.

Another object of the invention is to provide a tube for a tuned radio frequency amplifier stage, which tube is of the type including a positive screen grid and suppressor grid between the input grid and output plate, but wherein the transit angle between the screen grid and suppressor grid is suificiently large to produce a substantial phase shift in the transconductance of the tube.

Still other objects of the invention are to improve generally the efficiency and stability of tuned radio frequency amplifier stages, and more especially to provide a stabilized tuned radio frequency amplifier which will be economical in construction.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing, there is shown a stage of radio frequency amplification which includes a tube l 5 of the pentode type; the tube including a cathode 2, a signal input grid 3, a positive screen grid 4 and a plate 5. In such type of tube a suppressor grid, at cathode potential, is disposed between the screen grid 4 and the plate 5. Between the input 10 grid and cathode there is connected the resonant input circuit 6 and the source of signal grid bias 1, both arranged in series. Between the cathode and the plate of the tube there is arranged the positive energizing source 8 for the plate and a resonant output circuit 9. Variable condensers in the input and output circuits 6 and 9 respectively are employed to adjust the tuning of these circuits to any desired common radio frequency. A source of radio frequency voltage may be coupled to the input circuit 6, and any desired type of output circuit may be coupled to the resonant output circuit 9.

It is to be understood thatthe circuit 6 and 9 may be tunable over any desired range of radio frequencies. For example, a broadcast range of 500 to 1600 kc. may be the frequency range through which the amplifier stage is tunable. Furthermore, the stage to be described can particularly well be employed in the higher frequency ranges. The dotted line condenser l5 denotes the input grid to plate capacitance through which radio frequency feedback occurs when the phase of the plate current flowing through the output circuit is correct to cause regenerative feedback through the capacitance in into the input circuit 6. In such case the amplifier may not be stable. Even in the case where amplifiers, such as those of the pentode type, are employed with small input grid to plate cap-acitance, and shielding is employed, there may be sufiicient spurious capacity between the grid leads and the plate leads to produce sufficient magnitude of capacitance I!) to cause instability.

At high frequencies, because of the relatively slow passage of electrons from the cathode 2 to the plate 5, the transfer admittance acquires a phase shift. It is pointed out that with conventional tubes at medium frequencies the transfer admittance of the tube, which is equal to the rate 5() of change of plate current with control grid voltage, is a real quantity denoted by the symbol gm The transfer admittance may be expressed in general by the equation ZIm Q'm-I-jbm. 7 At ordinary wavelengths, the rate of change of Ib with phase in the output 156 is defined as gm. As the bias is increased, for example, the plate current decreases, practically simultaneously. At shorter wavelengths, the decrease is not instantaneous, but takes a definite time. This lag is taken into account by defining a new transadmittance: 11111. We can resolve ym into two parts: an iii-phase component, gm, and an out-of-phase part, bm. I have found that the stability of the amplifier is a function of the phase shift of the transfer admittance, which is equal to are a For values of this last quantity which lie between zero and ninety degrees the stability continually improves. For example, at 30 degrees the maximum tolerable feedback capacitance can be doubled. With ninety degrees phase shift no finite feedback capacitance will make the amplifier unstable. At the same time the non regenerative gain of the stage remains unaltered. If the phase has some other value, then there will be an addition to the overall gain due to regeneration.

In order to produce such a phase shift in the transconductance or transfer admittance, the transit angle between the screen grid 4 and the plate 5 can be increased; This may be done by a sufliciently large inter-electrode distance between the screen grid 4 and the suppressor grid ll. Again, it is also possible to employ two suppressor grids H and II between the screen grid 4 and the plate 5 so as to increase the transit angle, or increase the time of flight of the electrons to the plate 5. Additionally, lowaccelerations or voltages may be employed to increase the time of flight of electrons to the plate 5 and to secure the desired phase shift in transconductance. Of course, low accelerations and large inter-electrode distance between screen grid and suppressor grid Il may beused in combination to secure the desired phase shift in the transfer admittance. The suppressor grid II should preferably be close to the plate 5, so that the tube output conductance is not appreciably increased at high frequencies.

When the phase of the transfer admittance of the amplifier tube is shifted, the plate current circuit 9 is shifted so as to .decrease the regenerative feedback through the capacitance I0. With sufiicient phase shift of the transconductance, and subsequently the output plate current, the magnitude of the capacitance H] can be disregarded as the amplifier will have the desired stability. Because of this fact it is possible to construct the amplifier stage insuch tion.

a way that larger values of capacitance [0 can be tolerated. For example, as in the above, the shielding in the stage need not be excessive when employing only the present invention.

'It is to be clearly understood that the present invention is not .dependent upon any specific tube construction for producing phase shift in the transconductance, as it is contemplated that any device employed for producing such phase shift is encompassed within the scope of this inven- From a generic viewpoint, the present invention provides a method of improving the stability of a tuned radio frequency amplifier without in any way changing the actual magnitude of the input grid-plate capacitance,but by producing a phase shift in the transfer admittance of the amplifier tube to an extent sufficient to prevent radio frequency regenerative feedback through the aforesaid capacitance. Of course, the transfer admittance phase shift magnitude will depend upon the conditions encountered in practice. The direct current potential on grid H may be varied manually, or even automatically, so that the time of flight of the electrons can be altered as the frequency of the transmitted signals changes. 'The arrow 20 will be understood as generally comprising such a bias varying means. Those skilled in the art will readily be able to choose the constants .of the amplifier stage so as to improve stability by my present method.

while I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is: 1. A method of improving the stability of a tuned radio frequency amplifier which includes the steps of impressing radio frequency currents to be amplified upon an amplifier having input and output circuits tuned to the frequency of said currents, and shifting the phase of the amplifier tube transconductance to an extent sufficient to prevent regenerative feedback through the input grid-to-plate capacitance of the tube.

2. A method of operating an amplifier stage of the type including an amplifier tube having input and output circuits resonated to a common radio frequency, and said amplifier tube having a control grid to plate capacitance of predetermined magnitude, the method including the step of increasing the time of flight of the electrons to the amplifier output electrode to an extent suflicient to cause the amplifier transconductance phase to shift and thereby prevent regenerative feedback through said capacitance.

3. A tuned radio frequency amplifier of the type including an amplifier tube provided with at least a cathode, a control grid, a plate, a positive screen disposed between the control grid and plate, and a suppressor grid at cathode potential disposed between the screen and plate, a resonant input circuit connected between the cathode and control grid, a resonant load circuit coupled between-the cathode and plate of said tube, and the geometric relations between the screen and said suppressor grid being chosen so that the transit angle therebetween is sufficiently large to cause a substantial phase shift in the amplifier tube transconductance whereby regenerative feedback through the control grid to plate capacitance is substantially minimized.

4. A tuned radio frequency amplifierof the type including anamplifier tube provided with at least a cathode, a control grid, a plate, a positive screen disposed'between the control grid and plate, and a suppressor grid at cathode potential disposed between the screen and plate, a resonant input circuit connected between the cathode and control grid, a resonant load circuit coupled between the cathode and plate of said tube, the geometric relations between the screen and said suppressor grid being chosen so that the transit angle therebetween is sufficient- 151' large to cause a substantial phase shift in the amplifier tube transconductance whereby regenerative feedback through the control grid to plate capacitance is substantially minimized, and a decelerator electrode between the screen grid and suppressor grid.

5. A tuned radio frequency amplifier of the type including an amplifier tube provided with at least a cathode, a control grid, a plate, a positive screen disposed between the control grid and plate, a suppressor grid at cathode potential disposed between the screen and plate, a resonant input circuit connected between the cathode and control grid, a resonant load circuit coupled between the cathode and plate of said tube, the geometric relations between the screen and said suppressor grid being chosen so that the transit angle therebetween is sufficiently large to cause a substantial phase shift in the amplifier tube transconductance whereby regenerative feedback through the control grid to plate capacitance is substantially minimized, a decelerator grid between the screen and suppressor grids, and means for varying the decelerator grid bias to control the time of flight of electrons between the cathode and plate.

6. In a tuned radio frequency amplifier tube stage, the tube including a positive screen grid and a suppressor grid at cathode potential between the input grid and the output plate, the step of producing a phase shift in the tube transconductance by increasing the transit angle between the screen and said suppressor grid.

7. The method of increasing the stability of a tuned radio frequency amplifier tube by shifting the phase of the amplifier tube transconductance by such a value that no finite feedback capacitance Will make the amplifier unstable, while keeping the non-regenerative gain of the amplifier unaltered.

BERNARD SALZBERG. 

